Diazo-compounds are remarkably versatile intermediates in organic synthesis, participating in a variety of thermal, photochemical and metal-catalyzed rearrangement, addition, cycloaddition and insertion reactions, typically with concomitant expulsion of N2.[1] They have been found in nature, examples of which include azaserine[2] and members of the kinamycin[3] and lomaiviticin[4] families of marine natural products. Depending upon their stability and coexisting functional groups, diazo-compounds can present a challenge with respect to their preparation and isolation. Current methods include (a) diazo-transfer,[5] (b) diazotization,[6] (c) decomposition[7] or oxidation[8] of hydrazones, (d) rearrangement of N-alkyl N-nitroso-compounds,[9] (e) fragmentation of 1,3-disubstituted alkyl aryl-triazenes,[10] and (f) elaboration of more readily available diazo-compounds (FIG. 1).[11]
The preparation of diazo-compounds via the fragmentation of triazenes is uncommon. This route was described originally by Baumgarten, who isolated ethyl diazoacetate by the acid-catalyzed fragmentation of an aryl-triazene derivative (Eq. 1).[10a] More recent work by Bräse demonstrated that similar solid-supported triazenes, specifically those substituted with electron-deficient aryl groups, also undergo fragmentation under basic conditions (Eq. 2).[10b] The triazene precursors can be prepared by the addition of nitrogen-based nucleophiles to aryl diazonium salts or by the addition of organometallic species to azides with subsequent trapping of the resulting triazenyl anion with electrophiles, the former being the more popular approach.[12] These methods have limited synthetic utility.

The present invention provides a convenient synthetic route converting azides to diazo-compounds employing a tertiary phosphine reagent under mild conditions. The method is believed to proceed via formation of an acyl triazene which undergoes fragmentation in situ to form the diazo-compound in high yield. Certain acyl triazenes can be isolated and have been found to undergo thermal or base catalyzed fragmentation to diazo-compounds.